The prior art is replete with numerous examples of hot melt adhesives which are employed for the construction of disposable soft goods. Specific applications for these prior art adhesives have included disposable diapers, sanitary napkins, surgical drapes, hospital pads and adult incontinent products to name but a few. Moreover, the prior art methods of application of these prior art adhesives have included, but are not limited to, extrusion (multi-bead or slot), and spray or wheel application systems.
Those skilled in the art will readily recognize that many different polymer bases have been used, heretofore, to formulate hot melt adhesives for the construction of disposable soft goods. In this regard, the first copolymers to be employed were the ethylene vinyl acetate copolymers (EVA) and amorphous polypropylene (APP). While these polymers, when properly blended, provided acceptable adhesion to most substrates, they had several shortcomings which detracted from their usefulness. One of the first shortcomings of these polymers was that they lacked the desired elevated temperature resistance. For example, it is very important that a construction adhesive, for disposable soft goods, maintain its bond, not only at room temperature, but also at elevated temperatures, that is, 100° F. (38° C.). This elevated temperature resistance is important because without this characteristic, delamination of the end product occurs if the adhesive bond comes into contact with the user's skin. A second shortcoming of the prior art adhesives which were based in whole or in part on EVA or APP is that these polymers have a tendency to “gel” or otherwise increase in viscosity, or char when subjected to typical commercial application temperatures, that is 300°-350° F. In most instances, this drawback manifests itself in the form of poor application characteristics, such as plugged equipment nozzles. Further, those skilled in the art will recognize that adhesives based upon EVA or APP cannot generally be formulated as multi-purpose adhesive compositions.
As should be understood, multi-purpose adhesive compositions are those adhesives which can be used for more than one application. For example, and in the manufacture of most disposable diapers today, it should be understood that several different adhesive applications are present. These adhesive applications include the use of adhesives in construction, that is, bonding the polyethylene to the nonwoven and absorbent pad; the use of adhesives for elastic attachment, that is, bonding the elastic material to the polyethylene in either the leg and/or waist area; the use of adhesives for landing strips, that is, bonding a reinforcing layer of polyolefin film to the polyethylene in the area opposite the tape tabs; and the use of core adhesives, that is, applying an adhesive to the absorbent core to increase the strength of the core.
Construction adhesives are soft (tacky), have high peel strength, long open time and low cohesive strength. Elastic attachment adhesives are stiffer (not as tacky), high in cohesive strength, and shorter in open time, than construction adhesives. For multipurpose applications, in particular, elastic attachment, the balance of stiffness, cohesive strength, and open time is required for good performance.
As discussed earlier, the prior art EVA and APP based adhesive compositions have not generally been compounded, heretofore, to form multi-purpose adhesives with pressure sensitive properties because these formulations were found to be severely compromised as to the strength, and elevated temperature resistance. As should be understood, pressure sensitivity is extremely important to multi-purpose adhesive compositions because a pressure sensitive adhesive will form a bond over a much wider range of application conditions, such as temperature, than a non pressure sensitive adhesive composition.
Adhesive compositions based upon styrene-isoprene-styrene (SIS) block copolymers have also been used, heretofore, in the construction of disposable soft articles. SIS is chosen because, when compared to other block copolymers, for the same melt index and rubber content, SIS polymers provide a higher molecular weight and softer adhesive products. Adhesives used for elastic attachment need to contain high amounts of rubber and endblock resin to obtain adequate cohesion. For example, U.S. Pat. No. 5,149,741 to Alper discloses elastic attachment adhesives comprising 35 parts of SIS copolymer, in combination with 10 parts endblock resin. While these adhesive compositions have been used, they also have had several noteworthy deficiencies which have detracted from their usefulness. For example, adhesive compositions employing previously commercially available SIS copolymers displayed a low modulus and poor elevated temperature resistance even when formulated with various tackifying resins. In an attempt to improve the temperature resistance of the various SIS based compositions, endblock reinforcing resins were blended therewith. However, these resins appeared to decrease the specific adhesion of the adhesive compositions to polyolefin substrates and also raised the raw material cost of the final adhesive composition inasmuch as these reinforcing resins are generally quite expensive. Moreover, it appeared that, with respect to maintaining any significant degree of elevated temperature resistance, adhesive compositions containing SIS copolymers require relatively non-polar tackifying resins, that is, partially or totally hydrogenated resins or aliphatic C-5 resins. However, it became evident following experimentation that resins containing a significant amount of polar or aromatic components tended to associate with the endblock of the block copolymer, and thereby decreased the elevated temperature resistance to unacceptable levels. As a result, and when adhesive compositions were compounded using SIS copolymers and these relatively non-functional resins, the resultant adhesive compositions had what was considered very poor adhesion to polyolefin substrates. While these same adhesive compositions were acceptable for use as elastic attachment adhesives, that is, where a high level of specific adhesion is not generally needed, they further appeared completely unsuitable for use as construction adhesives, that is, where a very high level of specific adhesion to polyolefin substrates was mandatory. Moreover, these same compounds tended to yield products with undesirably high viscosities at application temperatures. As a result of the foregoing, adhesive compositions based upon SIS copolymers are not always suitable for use as multi-purpose adhesives.
In addition to the prior art adhesives discussed above, adhesives based on styrene-butadiene-styrene (SBS) multi-block copolymers were suggested for use in the construction of disposable soft goods. An example of these prior art adhesives is disclosed in U.S. Pat. No. 4,526,577. The SBS based adhesive compositions appeared to be improvements over the previously employed adhesives in several important respects, but they also had several drawbacks which detracted from their usefulness. For example, it was discovered that when an SBS based adhesive composition was left in an adhesive applicator for an extended period of time, it would rapidly increase in viscosity and ultimately gel thereby making it extremely difficult to remove. In addition, adhesive compositions based upon SBS multi-block copolymers did not have sufficient elevated temperature creep resistance to perform well as an elastic attachment adhesive as compared with adhesive compositions based upon the styrene-isoprene-styrene (SIS) block copolymers. Furthermore, many diaper manufacturers have recently added an elastic waistband to their disposable diaper products, and the SBS based adhesive compositions disclosed in this patent appear completely unacceptable for this particular manufacturing application. This is due to insufficient elevated temperature creep resistance.
In order to provide a higher level of creep resistance, U.S. Pat. No. 6,391,960 describes the use of an endblock resin in combination with a relatively high molecular weight radial or linear SBS copolymer as a multipurpose hot melt adhesive. It is stated therein that the presence of the endblock resin provides for a higher cohesion at lower viscosity, when compared to adding more copolymer to the composition to obtain the same increased level of cohesion. It is also stated that the presence of the high molecular weight SBS copolymer allows use of a lower amount of the copolymer which provides a softer, more pressure sensitive adhesive, with longer open time, when compared to use of a low molecular weight SBS copolymer. However, the use of these high molecular weight polymers leads to finished hot melt adhesives with unacceptably high viscosities.
It is desirable to keep the viscosity of the hot melt adhesive low to allow for low application temperatures, particularly when using spray equipment. This is especially necessary when applying the adhesive to heat sensitive substrates, such as the polyethylene backsheet typically used to manufacture disposable diapers.
Low viscosity hot melts can be made by using relatively low molecular weight linear SBS polymers, as described herein. The advantage of using lower molecular weight polymers, compared to the high molecular weight polymers used in U.S. Pat. No. 6,391,960, is lower viscosity of the finished adhesive, all else being equal.
However, as the overall polymer molecular weight decreases, it becomes more difficult for the endblock resin to stay incorporated in the styrene domain of the polymer. This is because as the polymer molecular weight decreases, so does the molecular weight of the styrene domain.
Therefore, it is desirable to keep the molecular weight of the styrenic endblock of the polymer as high as possible relative to the overall molecular weight of the block copolymer. This in part is achieved by keeping the styrene content high in the SBS block copolymer, for example, higher than 35 percent by weight.
Suitable endblock molecular weights can also be achieved when using linear block copolymers instead of radial. At a given polymer molecular weight, linear block copolymers have a higher relative endblock molecular weight than a corresponding radial polymer. For example, the general structure of a linear block copolymer made via a coupling reaction would be A-B—X—B-A, where A is styrene, B is butadiene and X is a di-functional coupling agent. If a similar radial polymer was made using a tetra-functional coupling agent, the structure would be (A-B)4—X. In this example, the molecular weight of the linear polymer would be twice that of the radial polymer. At very low polymer molecular weights, for example, where the toluene viscosity (25 weight percent polymer in toluene) is less than 1000 centipoise, the beneficial effect of aromatic endblock reinforcing resins would therefore be more pronounced in linear versus radial SBS polymers.
The unique combination of low polymer molecular weight combined with high styrene content and linear polymer architecture results in a low viscosity hot melt adhesive with excellent elastic attachment performance.
As used herein, the “midblock” of the polymer refers to polymeric blocks which are substantially aliphatic. As will be discussed below, “midblock resin” refers to a tackifier which is compatible with the midblock of the polymer. “Endblock” of the polymer refers to polymeric blocks which are substantially aromatic. “Endblock resins,” as will be discussed in detail below, are substantially aromatic and compatible with the endblock of the polymer.
Malcolm U.S. Pat. No. 5,057,571 discloses an adhesive for elastic attachment comprising low content of a very high molecular weight SBS copolymers. However, these very high molecular weight polymers can be very difficult to compound in typical hot melt production equipment. In addition, at very low polymer concentrations, the adhesive tends to become “glassy” since very high resin loadings are required.
As discussed above, endblock resins are commonly used with SIS to improve cohesive strength in pressure sensitive adhesives. Endblock resins are not commonly used with SBS because it is difficult to ensure that the resin actually incorporates in the endblock due to the higher polarity of the butadiene midblock compared to isoprene midblock, which therefore tends to solubilize the endblock resin in the midblock to a large extent. In addition, endblock resins are not commonly used in SBS-based elastic attachment adhesives. However, Raykovitz U.S. Pat. No. 4,944,993 discloses low molecular weight, radial SBS polymers comprising a styrene content greater than 35% with an endblock resin, and their use in construction and elastic attachment adhesives.